Comparative study of hand layup and automatic fibre placement

Comparative Assessment of Hand Layup and AFP Harry Clark,1 Ben Joseph,1 Emer Brolly,2 Lorenzo Ceresole,2 and Sam Retford2 2

1 Engineering Design, University of Bristol, UK Mechanical and Electrical Engineering, University of Bristol, UK

Automatic Fibre Placement (AFP) and Hand Layup are two open moulding lamination processes used to manufacture composite parts. This paper will discuss the history and technical details of each process in the introduction and overview. Following, a comparative assessment of various aspects of each process is evaluated, attaining key points of contrast between the manual and automated techniques. The areas of comparison are: material limitations, geometry requirements, part properties, costing, sustainability, safety and the potential future developments. This study found that a direct comparison between the two processes is essential for making rational decisions when choosing between the two for manufacturing purposes.

I. I.1.

INTRODUCTION

Automatic Fibre Placement

AFP is the computer controlled process of laying up a series of pre-impregnated fibre tows. It can be used to produce large composite parts with complex geometry and is commonly used in the aerospace industry to manufacture aircraft fuselages [23].

AFP was developed in the late 1970s by combining the processes of filament winding and automated tape laying (ATL). Originally, filament winding systems were converted to layup prepreg tows by applying pressure to the contact site and winding around a rotating mandrel [13]. In its early stages of development, AFP was a very expensive method of layup and could only handle a small number of tows, lending itself to military and aerospace applications due to the high cost of the composites at the time[24]. Developments in robotics during the 1980s resulted in large improvements in tow synchronisation, increasing the accuracy of the layup and reducing buckling and voids [24].

Figure 1. AFP system diagram [13]

In the 1990s, the use of a thermoplastic matrix was introduced, enabled by the development of on-site consolidation using hot gas torches [12]. On-site consolidation of carbon fibre reinforced polymer (CFRP) with a thermoplastic matrix removed the autoclave size constraint [24],

enabling the manufacture of much larger parts. During the 2000s AFP saw considerable development in its capability: increasing the number of tows laid simultaneously, increasing linear speeds and reducing cut-stop-restart downtime[24] and steering radii[32]. AFP systems could now handle geometry of greater complexity, at greater speeds, with less wastage and at a lower cost.

Today, AFP systems can deposit up to 32 prepreg tows simultaneously at individual linear speeds of up to 1m/s [24]. The tow width typically ranges from 3.175mm12.7mm [22] and is consolidated with a thermoset or thermoplastic matrix. Primarily the tow materials are limited to carbon fibre however, there has been recent development in bio-composite matting [6]. Pressure is applied to the contact site with flexible rollers to prevent the formation of voids between layers.

I.2.

Hand Layup

Hand layup is one of the oldest and most basic processes within the family of open moulding. It is a very technical and labour intensive process that requires skilled workers[32], however, it remains one of the cheapest processes for producing composites as there is minimal set up cost.

Woven glass fabric was developed by British silk weavers in 1842, by drawing thin fibres from a pool of molten glass and intertwined with silk cloth. Since the early 1930s, glass fibre has been produced through the extrusion of melted glass. This technique has been improved throughout the years and was first used for the production of composites in the latter years of the decade, leading to the first patents for thermoset resins in 1935 and composite boats in the following years[25]. Fibreglass has a wide range of applications, from automotive to architectural industries, and therefore production is large scale and significantly cheaper than carbon fibre.

In the early 1960s, boron fibre composites were introduced to hand layup and they became the basis for advanced composites. However, they were quickly replaced by carbon fibre. The introduction of carbon fibre greatly increased the speed of the hand layup process as it was

2 much easier to manipulate into complex geometry, such as concave shapes[18]. The need for high performance composites increased throughout the 1970s as the price of fuel rose and research increased with projects such as the Aircraft Energy Efficiency program[18]. The composites industry has been rapidly revolutionising and many other forms of composite layup have been developed, however, hand layup still remains a viable and important process for high performance industries.

There are two main forms of hand layup. The first involves a dry reinforcement fibre, which is placed into a mould and then a wet resin is applied over the top and consolidated with rollers to eliminate air bubbles between layers. The second is where prepreg fibres are placed onto the mould and consolidated in the same way. The process is versatile as it can be used to produce parts of a variety of sizes, from boat hulls and architectural mouldings to wind turbine blades [31]. Production volume is low due to the process being very labour intensive. This has resulted in a push for AFP to replace hand layup. As the speed, accuracy and reliability is improving, AFP is becoming more viable for general application [30].

by the compaction rollers induces matrix diffusion and eliminates voids and tow gaps [32].

A Cut-Stop-Restart system is used to start and end the tow paths [20], minimising material waste.The CutStop-Restart system also alters the number of tows deposited when steering the tow path [20]. Higher quality parts are achievable with uniform compaction pressure, as this increases the fiber steering flexibility, increasing the systems geometric capabilities and eliminates debulking processes[20]. Heat is transferred in to the tow and the nip point to reduce the matrix viscosity, and increase layer adhesion[12], this process is called the tack enhancement [20]. The moulds used in AFP are typically made from aluminium [32] as the metal draws heat away from the part and increases the rate of consolidation.

Figure 3. Large scale CFRP AFP lay-up [13]

II.2.

Figure 2. Hand layup diagram [2]

II. II.1.

PROCESS OVERVIEW Automated Fibre Placement

An AFP system consists of 3 major parts: the gantry - responsible for depositing the fibre tows, the robotic arm - responsible for moving the gantry and the creel - a rack of bobbins storing the tows. The gantry, as shown in Figure 1, is connected to the top of a robotic arm, fed by by a creel storage system. The gantry contains: compaction rollers, a heat source and the tow delivery system. Up to 32 pre-impregnated composite tows are run from bobbins housed in the creel to the compaction roller through a series of tensioning rollers [13]. Each tow is individually tensioned to aid tow steering. To deposit the fibre tows the backing paper is removed and stored on a separate reel in the gantry, the area in front of the nip point is heated [23], then pressure is applied to the nip point by the compaction rollers. The pressure applied

Hand Layup

The hand layup process consists of three main stages; coating, layup and curing. First, the mould is cleaned and then coated with a wax or solvent release agent, this aids the part’s removal by preventing the gel coat from adhering to the mould. The gel coat applied, provides a hard, high sheen finish to the outside of the part, it is sprayed on to achieve a uniform coat thickness. The fibre reinforcement layers are then laid up and consolidated with a resin matrix using squeegees and rollers. Deposition and consolidation is repeated until the required part thickness is achieved. The finished part is then left to cure at room temperature. For high-performance composites curing may require the application of heat and a reduction in pressure in an oven or autoclave. Glass Reinforced Plastic (GRP) can be cured at room temperature or in an oven, however, autoclave heating is essential for Carbon Fibre Reinforced Plastic (CFRP) curing[9].

In hand layup, female moulds are used for applications such as boat hulls to give a high quality, smooth finish to the exterior of the hull. Male moulds are typically used for applications such as baths where a smooth internal finish is required. The release agent used depends on cost and cycle time. Semi-permanent solvent based release agents, although more expensive, cure quicker and one application to the mould can last multiple cycles. This makes it

3 the standard in high performance composite implementations such as aerospace and motorsport [33]. The gel coat is usually 0.03 inches and in certain compositions can add mechanical benefits such as malleability and toughness or physical properties such as weather resistance[10].

However, if the coat is too thick then it can make the part brittle [10]. Pre-impregnation commonly takes place before deposition. The fibre matting and resin are rolled between two sheets of plastic to achieve uniform matrix diffusion. This process also negates the propagation of voids by forcing out air bubbles through compression as further discussed in V. The most basic option for curing is at room temperature but this can be accelerated using alternative process.

When heat is applied, its important that the "ramp up" of temperature is increased in small increments over the coerced time period specific to the resin used. The same goes for the "ramp down", otherwise distortion and warps can occur[3]. If high performance thermosets are used the parts must be cured in an autoclave, where parts are subjected to heat and high pressure. As this is an expensive process parts are usually cured simultaneously, with computer systems monitoring the process[3]. Alternate methods are available, by using microwaves, X-ray or electron beams on radiation sensitive resins, polymerization and cross linking occur from the ionizing radiation. By adding a photoinitiator to a thermoset resin UV light can initiate a crosslinking reaction. [3]

Figure 4. Goldsheild Fibreglass Hand Layup Diagram [4]

III.

MATERIAL LIMITATIONS

The composites used in both hand layup and AFP comprise of two main constituents, the core and matrix.

The core is a matting or fabric woven from microscopic glass [19] or carbon fibres[24]. The matrix is a thermoset polymer, Thermoplastic polymer or Dry Binder[12] which are consolidated with the fibre matting using pressure applied by a compaction roller and cured to produce a fibre reinforced plastic part[13].

In AFP, to consolidate the tow being laid with the tow below, heat is applied in front of the nip point by a Infrared (IR) lamp [23], Hot Gas Torch (HGT) or Fibre-Couple Diode Laser [12], as described in Section II.1. These heat sources will provide enough energy to lower the viscosity

of the matrix to achieve full consolidation and matrix diffusion or full binder activation [12]. Historically, Hot Gas torches have been used because they are relatively inexpensive and transfer heat to the tow at a higher rate[12]. However, more recently, Diode laser systems and IR heat lamps have become common place as they have a lower temperature response time [12] and propagate less heat to the area surrounding the nip point[23].

Matrix

Consolidation Temp. (°C)

Epoxy Polypropylene (PP) Polyamide (PA) Polyphenylene Sulphide (PPS) Polyether Ether Ketone (PEEK)

20 - 70 130 - 200 130 - 200 280 -350 280 -350

Table I. Comparison of Matrix Consolidation Temperatures for AFP [12]

Table I compares the consolidation temperatures required for AFP tow deposition. When using a thermoset resin, the part will cure via chemical reaction often aided by an oven or autoclave [6]. When using a thermoset resin in an AFP system, like Epoxy or Polyester Resin, a low consolidation temperature is necessary because the resin is already tacky at room temperature [12]. Heat is applied only to increase the adhesion and lower the matrix viscosity [12]. During hand layup, however, no heat is applied to the consolidation site. It is not necessary for this process as the variations in the properties of the prepreg matting can be accommodated for by the technician during layup.

The use of a Thermoplastic and Dry Binder matrices is limited to AFP systems, as consolidation requires the application of a controlled heat to bring the matrix above the glass transition temperature Tg while remaining below the matrix melt temperature Tm [6]. As shown in Table I the consolidation temperatures required for thermoplastic polymers matrices range from 130°C − 350°C. To consolidate a Dry Binder matrix a controlled heat must be supplied well above the binder activation temperature, this is typically done with a laser heating system due to the slower transfer of energy. This ensures full binder activation [12].

Diode Laser, IR lamp and HGT heating systems cannot be used by a technician with the precision necessary for even heating. Therefore, the consolidation temperatures for Thermoplastic and Dry Binder Matrices cannot be reliably achieved during hand layup, limiting the range of feasible matrices for this process to thermoset Polymers.

IV.

GEOMETRY REQUIREMENTS IV.1.

Shaping Prepreg

When placing an inextensible sheet onto positive and negative gaussian curves, material excesses and deficien-

4 cies are created. The fibres of a sheet are inextensible but the woven sheet can be sheared to fit complex geometry.

Shearing is creating tension in the fibre to cause to sheet to increase in size at 45°to the fibres while also reducing in size in the perpendicular direction to the tension applied[14]. Therefore, there is a need for shearing to be applied to the prepreg in order to help it fit to the mould. This avoids the use of slits and folds to preserve the mechanical properties of the composite. The shearing required for the prepreg greatly increases as the angle of the ramps used in the mould increases. This increases the complexity of the hand layup process and presents more opportunities for bridging and wrinkling to develop in the prepreg due to the higher forces required to shear at larger angles[14]. The forming of these imperfections leads to weakening of the finished part in localised areas.

IV.2.

Deposition Limitation

Hand layup is a manual process, therefore, there can be much greater variations in the size of the part and complexity of geometry when compared to AFP. A technician can accommodate for much smaller geometry as the consolidation pressure is applied by hand. However, the geometry achievable in AFP is limited by the Fibre Steering Radius Rs [32]. Rmin is the minimum radius of the curve of tow deposited without part defects. The Fibre Steering Radius achieved depends on factors such as consolidation temperature, compact pressure and the speed of deposition [32]. If a curve is laid where Rs < Rmin buckling and voids can occur, dramatically reducing the structural capabilities of the part. Rmin for 1/8 in CFRP epoxy tow is 508mm [24]

Figure 5. 1/4 in CFRP tows deposited on woven Carbon Fibre matting with an applied temperature of 150°C. [32]

Figure 5 shows that tow paths of a smaller steering radii of <1250mm can incur tow overlap and buckling in 1/4 in tows [32]. This effectively limits the geometric capabilities of AFP. Hand layup has not such limitations because a technician can use a smaller diameter roller to accommodate for complex geometries.

V.

PART PROPERTIES

The quality of a composite part is primarily affected by two variables: the composite porosity and the density of the fibre matting used[21]. The porosity is determined by the concentration of air bubbles in the resin and void spaces between the layers. In hand layup, void formation is caused by insufficient fibre impregnation or compaction pressure applied during layer consolidation [27]. However, during AFP, void formation is caused by insufficient heating and compaction pressure during tow deposition.

High porosity leads to an increased strain on the fibres and lower load capacity. Matrix properties such as shear strength, longitudinal compressive strength and transverse tensile strength are also affected, as shown in Table II [29] [27].

Property

∆void (%)

∆property (%)

Shear Strength Compression Strength Tensile Strength

+1 +1 +1

reduced ∼ 3 reduced 5 − 10 reduced 10 − 20

Table II. Effect of Voids on Mechanical Properties[27]

The fibre volume ratio of the composite has a direct correlation to the overall strength of the part[21]. The maximum fibre volume ratio attainable during hand layup is ∼ 35%, as opposed to the attainable volume ratios of other methods of composite manufacture which can reliably achieve >50% [8] [16]. The low volume of fibre causes a reduction in strength due to the low density of the fibre mesh which reinforces the structure.

During hand layup, the resin is forced into the fibre matting by using rollers and pressure applied by hand.

This process consolidates the fibre layers and reduces the porosity of the composite. Compaction can also be carried out using vacuum bag to attain a more uniform layer thickness, resulting in better fibre volume ratios[10].

However, the automated nature of AFP allows for larger compaction pressures with greater precision. Therefore, AFP systems are capable of greater levels of consolidation and a higher fibre volume, leading to parts produced by AFP having better mechanical properties than their hand layup counterpart.

VI. VI.1.

COSTING Set-up Costings

The initial set-up cost of AFP is much higher than that of hand layup, this is due to the inherent need to purchase an AFP machine. In 2013, the AFP system at National Composites Centre (NCC) in Bristol cost $4 million to install [26]. However, the AFP machine can reduce material waste by 75% when compared with hand layup [26], resulting in a cost reduction per part. Both

5 hand layup and AFP require moulds, which adds to the expense of both processes. However, unlike AFP, hand layup is labour intensive which increases manufacturing production costs, more so when the mould is complex [14].

VI.2.

Cost per Part

An economic model produced by Bauswein, Veldenz and Ward [7] provides an accurate cost range for the price per part of AFP and hand layup depending on the input parameters [7]. In the model, test parameters were set to produce 25 identical parts with area 0.22m2 over the first year. The set-up, material and labour costs were incorporated into the model. Under these parameters, Figure 6 compares the part cost when using AFP and hand layup with various curing processes.

produced each year effects the cost per part and how the price part part changes with each year of manufacture.

Figure 7 only compares hand layup to dry fibre Afp, both cured in an autoclave, with the same part parameters as figure 6 [7].

From this model its clear that in both processes there is a decrease in cost per part with each manufacturing year. Along with a logarithmic decreases in the cost per part when a larger volume is produced [7]. Although AFP costs do decrease, throughout all ten years and production volumes, the part cost is always greater than hand layup at its most expensive (1st year, 25 parts, £482/part)[7] .

This model indicates hand layup is a cheaper manufacturing option for a composite part this size, and why its still widely used throughout the composite open mould industry.

VII.

Figure 6. Estimated production costs of different manufacturing options for a 32 ply thick part[7]

The model states that for the first year of manufacture, AFP part costs can range from £522 − £658 [7], with the dry AFP process being cheaper than prepreg AFP for a part 32 ply thick. But there is a significant 9% cost reduction [7] between dry AFP and the hand layup process. The hand layup process part cost was much lower, £476 − £482 [7]. In this case,hand layup is the cheaper method for producing composite parts of a 32 ply thickness.

Figure 7. The influence of production volume on production costs, comparing hand layup with dry fibre AFP [7]

Figure 7 goes on to compare how the volume of parts

SUSTAINABILITY

When considering sustainability the largest factors are the fibre matting and matrix, as the variation in the material of these constituents determines whether the part can be recycled. When thermoset polymers are cured, the chemical structure changes within the polymer and the reaction is irreversible[6]. Therefore, thermoset plastics cannot be melted and reformed in the same way thermoplastics can. The irreversible nature of thermoset curing makes recycling fibre reinforced thermosets much harder than fibre reinforced thermoplastics.

VII.1.

Thermoset Polymer Matrix Recycling

Since the 1970s, Glass reinforced plastic (GRP) parts have been processed using mechanical grinding. At the end of its life a part is cleaned shredded and ground into a fine powder. This powder can be used as a filler in new GRP parts in quantities up to 20% of the total composite weight [19]. However, the processed GRP is likely replacing a calcium carbonate filler which costs ∼ £200/tonne [19]. This does not lead to significant savings considering the costs associated with cleaning and grinding the GRP waste.

In the early 2000s, there were developments in the use of Pyrolysis to separate the glass fibre from the thermoset resin [19]. The matting and resin can be separated by Pyrolysis due to the significant difference in degradation temperatures of the silicate-based glass and organic resin.

A study by The University of Leeds at this time showed that glass fibres processed using Pyrolysis to replace virgin glass by quantities up to 20% [19]. While mechanical grinding is used in industry today, neither process is economically viable due to the energy costs of processing and relatively low product return.

As discussed in Section III the process of hand layup is limited to produce composites with a thermoset resin.

6 However, AFP is not, leading to greater opportunity for recycling and reprocessing.

VII.2.

Thermoplastic Polymer Matrix recycling

The use of a thermoplastic matrix can lead to a reduction in part and material stock waste as they have an "unlimited" shelf life if stored correctly [6]. An inherent property of thermoplastics is the ability to be remelted and reformed. This allows the part to be reheated and re-consolidated post forming, providing the opportunity for a technician to fix voids and defects that would have otherwise caused the part to be rejected [6].

Recent developments in use of Polysaccharide-based Flax Fibres and Polypropene (PP) tape during AFP manufacture[6] could lead to commercially available, biodegradable composites. If additives are incorporated into some thermoplastics like PP the rate of bio-degradation can be increased. These additives dramatically reduce the time taken for discarded thermoplastic parts to degrade naturally by increasing the plastics reactivity with oxygen, water and UV light.

There is a significant difference in the melting temperature of thermoplastics matrices, such as PP and Polyamide (PA), and the glass or carbon fibres with which they are consolidated. This means that it is be possible to melt clean waste composite at a temperature above the plastic melt temperature and below the fibre melt temperature.

Therefore, it is possible to separate fibre and matrix for reuse at temperatures above ∼ 200°C[12]. However, this separation is not possible for hand laid parts due to the use of a thermoplastic matrix, as discussed in Section III. VIII.

SAFETY

During the process of both AFP and hand layup precautions must be taken to ensure the technicians’ safety.

Especially during finishing processes as fibre particulates can be created and released in quantities hazardous to health [34].

During hand layup, respirators can prevent contamination of the part as well as ensuring micro fibres are not inhaled. The risk of inhalation is further reduced by factory ventilation [11]. Latex gloves are worn to ensure the fibres do not stick to the skin, but if there is a risk of fibre splintering, heavy gloves must be used. The fibres used as matrices are mechanical irritants to the eyes and skin and the Volatile Organic Compounds released by the thermoset resins are toxic if inhaled [1]. To improve handling, both carbon and glass fibres may be coated in a sizing material, which is generally a form of epoxy resin.

Due to the automation and remote operation of AFP, the above hazards are less of a problem. Nevertheless, there remains several hazards that can cause injury or malfunction during system operation. Human errors occur through incorrect programming or entering the system’s

safeguarded area during operation requiring an emergency stop. Also, electromagnetic or radio frequency interference may cause control errors. Incorrect system installation and mould defects can lead to mechanical failures during operation. The above hazards can lead to injury and loss of life, as well as part defect. [17] IX. IX.1.

FUTURE ADVANCEMENTS Automated Fibre Placement

In the industry developments, AFP is focused around increasing the overall productivity of the process. Currently research is being conducted in computer simulation, machine optimisation and materials properties.

Software developments are focused on path optimisation to reduce the travel time between tow paths. These innovations have already lead to a 33% reduction in layup times[13]. The largest increases of productivity were seen on small parts due to the shorted parth lengths.

These innovations are similar to those being used in hand layup where computer programs suggest different starting points, resulting in different deformation patterns to improve cycle times[14]. These advancements also have the potential to reduce part defects such as tow misalignment, buckling and pull up.

Research into AFP dedicated prepregs could be beneficial as most manufacturers focus on optimising hand layup and defects in AFP lead to prolonged downtime as robots must be reset[13]. The curing process has also been subject to development. As the possible size of the AFP components grown hand layup has become less viable options. One of the suggested solutions is to use UV light to cure thermoset resins in-situ. In research it was found that the use of a polymer matrix comprised of an epoxy acrylate and cycloaliphatic epoxy resin, acheived an optimal degree of polymerization (67.7%) with an irradiance of just under 4mW/mm2 [5]. Currently, the process of in-situ UV curing insufficiently manages resin content. Therefore, The inter-laminar strength of the component is significantly lower than that of autoclaves cured composite due to the composition of the resin used to achieve a UV curable matrix. However, in-situ UV curing has the potential to provide a cheaper and more sustainable alternative to autoclave and oven curing.

IX.2.

Hand Layup

Automation of hand layup has the potential to greatly reduce the cycle time and replace the labour intensive part of the process. One form of automation was proposed

by Elkington, Ward and Potter at Bristol University. The idea is to mimic the process of hand layup using robotic arms which intends to bring together the speed and accuracy of AFP and the dexterity of hand layup [15]. This had been tried previously, but the complexity of the shearing

7 the field of composite production. However, in its current state the process is not fast enough and does not provide much advantage in quality of layup over that of a human.

X.

CONCLUSION

required while the layup process was ongoing required too many different gripping points for the robot[28]. Instead they looked at a different way of introducing the correct deformation to the prepreg by preshearing the material.

This was carried out by placing the prepreg onto a skeleton mould shape to apply approximate deformation to the prepreg with a low heat source to combat spring back.

This allowed the sheet to be laid up by a 6-axis robot with multiple end effectors. These end effectors each help tackle different shapes from flat surfaces to complex geometry such as double curvature saddles. The automatic layup is carried out from areas of high shearing to areas of low shearing which helps to reduce the chance of wrinkling. This process presents an exciting new direction for

Both AFP and hand layup are very useful within industry, from aerospace to prototyping. AFP is automated; therefore it can produce large products accurately, which can then be reproduced with a high level of precision.

However, it has very large-set up and maintenance costs.

Hand Layup is an established process which has a lower set-up and per part cost when compared to prepreg AFP [VI.2]. Hand layup is also preferable for intricate, complex parts, as AFP is restricted by the fibre steering radius [IV].

Hand layup has its restrictions too; the matrix materials used are limited to thermoset plastics. However, AFP can additionally utilise thermoplastics and dry binders, which are less toxic to the environment.

For the future in composite manufacturing, developments in AFP will lead to overtaking hand layup within industry, or both process will be replaced by automated hand layup. In both cases, future advancements will decrease the set up cost and increase the reliability and speed of the machine used [IX].

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